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Trowel on Vapors Retarders and Rh
02-07-2011, 07:45 PM
Post: #1
Trowel on Vapors Retarders and Rh
I recently tested a slab that was below grade and then after the Bostik MVP4 app. The rh went up about 6 percent afterwards. From everything I read that should not happen, should it? They were runnign high 40's and one a low 50rh.

Then I threw down three CaCl tests, one on top of the MVP4, one on top of six mil plastic thats covering most of the the slab and one cacl test surrounded by the plastic. The cacl tests ran 1.4, 0.5 and 1.5 respectively. The one on top of the plastic being the lowest.

Several days after I put down the 6 mil plastic the rh is still running the same, 55 rh only on the MVP4, 65 rh on the MVP4 surrounded by plastic, and the last one totally covered by plastic ran 50 rh which was next to the lowest cacl test on top of the plastic.

This part of a multi level house built up against the side of a mountain has water trickling out of the granite underneath the house at the lowest level down by the pool.

In your opinion would it be safe to install soild hardwood on that slab over the plastic and MVP4 on a floating plywood subfloor?

Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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02-08-2011, 05:57 PM
Post: #2
RE: Trowel on Vapors Retarders and Rh
Twenty hits and counting and no one wants to offer an opinion? C'mon man. The one wall is only 3 feet below grade. I think my system will work grat with any soild hardowwd below grade.

Plus I think the builder and architect did a fabulouso job with that slab.

Where's all the experts when you need them?

Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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02-08-2011, 07:24 PM
Post: #3
RE: Trowel on Vapors Retarders and Rh
Ernesto,

Those hits you see may not be people reading your post and ignoring you.... There are 'bots' and 'spiders' that crawl the internet looking at websites, touching on threads, searching for information.... Spooky huh???? So no disrespect, nobody is avoiding you!!! I hope... Tongue


I'm not completely sure if I follow you, but are you placing MVER kits on the plastic???? If so, that should read just about zero, but of course CaCl kits are prone to errors, so reading half a pound is possible.

Can the RH go up after a slab is sealed? Definitely!!! A slab that is fed moisture from below (violated vapor retarder (VR) or no VR, or a slab on a sand bed, or perhaps the VR is just a very high perm VR that allows a lot of vapor through) anyway, if moisture is being supplied to the underside of the slab faster than the top side is emitting it, well then the moisture content of the slab will most likely go up.

We see this a lot on remodel jobs where contractors trust a CaCl test to qualify an old slab and they may get an acceptable reading, unaware or maybe not even considering that the slab may be subject to subsoil moisture. They put down a vapor retarding flooring (which can be anything less breathable than what was there previously) and the moisture content of the slab starts to climb. Oftentimes the problem doesn't show up for a year or more, and the parties involved wonder where this mysterious water came from.

It was always there, but capping off the top, sealing off the only avenue the slab had to dry out before, revealed a deeper problem. This is where an RH test will help you see that the slab has moisture inside of it, and then you can determine where and why that moisture is there and take appropriate measures to deal with it. Wink

JD Grafton
Concrete Answers for Flooring Problems
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02-09-2011, 07:26 AM
Post: #4
RE: Trowel on Vapors Retarders and Rh
But but.....according to the graphs, if I am at 40% depth I should be in the highest point of the slabs moisture content. Yes or no? Then after it equilibrates to the surface where the mvp4 is or "vapor retarding cover" as you call it, it should remain the same.

Still three weeks later they are running the same as several days after installing the mvp4 and 6 mil plastic.

Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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02-09-2011, 08:39 AM
Post: #5
RE: Trowel on Vapors Retarders and Rh
The 40% depth rule is for a slab that has an intact and proper vapor retarder directly in contact with the bottom of the slab. In your specific situation, you may not have this.

If water is being supplied to the bottom of the slab, and the slab is open and drying to the top, the lower portion will be much wetter than the top portion. There would be no specific location to read a 'moisture equilibrium' level because theoretically the slab can become completely 100% saturated with water when the top is sealed. This is where many people get in trouble because they get a reading, apply a sealer or a floor covering based on that reading, and later down the road problems crop up and the slab is wet. Where did the water come from? From the moisture continuously supplied from below.

Sometimes this moisture takes a long time to manifest itself because of water tables, temperature changes etc. A slab can remain quite dry until a wet season or flood pushes water under it. This is not an issue with a good vapor retarder, so our testing and recommendations should always be based on having all the pieces of the puzzle in place.

If the slab has a proper vapor retarder, or is poured in an elevated pan, then the 40% depth will show you the correct slab RH.

JD Grafton
Concrete Answers for Flooring Problems
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02-09-2011, 10:40 AM
Post: #6
RE: Trowel on Vapors Retarders and Rh
Are you sure you are using rapidrh? I've never seen rH numbers that low in concrete.

I'd say you are running into a common issue when you put a membrane on any slab. The rH equilibrium across the slab. That is why if a floor covering calls for 75% and you install at 74% when you get your failure it will always be higher. Essentially that is what the bostic is doing. It is holding the moisture to allow the whole slab to reach equilibrium just as if you installed flooring. Perhaps someone has that graph handy to post that shows slab equilibration vs rH level over time? Looks like a slab with a T under it and a curved line?

As someone who works for a manufacturer we know if we say 75% what we are saying internally is closer to 80-85%. The less responsible manufacturers will say will say 75% as an out. Because they know 99% of installers will install @ 74% and they know that should a failure happen the 74% will get blown out of the water. (no pun intended)

This is even more significant with CaCl testing.
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02-09-2011, 06:06 PM
Post: #7
RE: Trowel on Vapors Retarders and Rh
The reason we measure at 40% depth is because extensive testing has shown that a slab will equilibrate to the same moisture level over time as we read at the 40% depth initially.

A bad vapor retarder will completely destroy our theory however, as it supplies an unending source of water.

JD Grafton
Concrete Answers for Flooring Problems
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02-09-2011, 07:42 PM (This post was last modified: 02-09-2011 07:46 PM by Ernesto.)
Post: #8
RE: Trowel on Vapors Retarders and Rh
Yes rapid Rh what else would I stick down a hole and use a reader, I got pictures and a reciept if you wanna see it.

Seeing that my readings have stabilized after the MVP4 went down I think the slab has a barrier under it. Plus the fact that twenty feet lower water was seeping out of the rock face and all the flagstone down there is still wet.

Those low numbers, initially high 40's and one a high 50's have been documented with a picture and a time date stamp. Since they went up after the MVP4 app, all I can summize is that the depth of the concrete may have been deeper than I was told.

But still, after four weeks and they are stable at 50's and one 65 rh I think...no positive it's a good read. Gotta love the fact that you can go back weks later and simply stick your reader in. Love the Rapid Rh Smile

Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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02-09-2011, 08:40 PM
Post: #9
RE: Trowel on Vapors Retarders and Rh
pictures of the job for everyone: http://www.flooringinstaller.com/forum/t...2#comments
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02-10-2011, 06:43 AM (This post was last modified: 02-10-2011 06:47 AM by Ernesto.)
Post: #10
RE: Trowel on Vapors Retarders and Rh
Thats a great thread, love it when Bob chimes in about admixtures and all that stuff. Makes you wonder who can predict if any slab is safe! Click on Bob and go to his page over there and you will find lots of threads he posted. I like the one about ph the best.

Didin't notice I got a smudge on the lens dang it.

Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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